This is mostly no longer needed. A negative value for plane is the preferred way to do show objects in the background. It can still be used however when you want to rearrange objects in the same plane when using PLANE_MASTER for visual effects.
EFFECTS_LAYER is a special high value that can be added to the regular layer of any atom.
The purpose of this value is to make an atom appear above any regular atoms. For instance, in an isometric map if you want to display a character’s name below them, it does not make much sense to have nearer objects cover up that name, so you can tell the name overlay to use EFFECTS_LAYER + MOB_LAYER and it will show up on top of all the normal icons on the map. This has been somewhat obviated by plane but may still be useful in some cases.
When using this special layer, it should be added to the layer an atom normally uses. For instance an obj should have a layer of EFFECTS_LAYER + OBJ_LAYER.
This can be mixed with TOPDOWN_LAYER, in non-topdown map formats. Anything in TOPDOWN_LAYER will display on top of EFFECTS_LAYER, and TOPDOWN_LAYER + EFFECTS_LAYER will be above both.
This can also be mixed with BACKGROUND_LAYER, which takes priority over everything else.
Images or overlays with FLOAT_LAYER can be left alone. They will automatically have the same layer as whatever atom they are attached to.